Bill Gates became rich primarily by co-founding Microsoft in 1975 with his childhood friend Paul Allen. The company developed and licensed software for personal computers, most notably the MS-DOS operating system, which Gates licensed to IBM for their PCs. Instead of selling the source code outright, Gates arranged a licensing deal that allowed Microsoft to license MS-DOS to other PC manufacturers, which was a pivotal business strategy that fueled Microsoft's rapid growth

. Microsoft went public in 1986, and Gates became a millionaire almost immediately. By 1987, when Microsoft’s stock price rose significantly, he became the youngest self-made billionaire at age 31. His wealth grew as Microsoft dominated the software market, especially with the success of the Windows operating system, which eventually achieved over 90% market share in personal computers

. Beyond Microsoft, Gates diversified his fortune through investments in various industries such as waste disposal and agriculture, and he owns a personal investment firm that focuses on healthcare, technology, education, and environmental sectors. Even after stepping down from day-to-day operations at Microsoft, he continues to earn significant income from his stock holdings and investments

. In summary, Bill Gates got rich by:

• Co-founding Microsoft and leading it to dominate the PC software market.
• Licensing MS-DOS to IBM and other PC manufacturers, creating a recurring revenue model.
• Taking Microsoft public in 1986, which greatly increased his personal wealth.
• Holding a large share of Microsoft stock as the company grew exponentially.
• Diversifying his wealth through investments beyond Microsoft